The present invention relates to a filtration module for crossflow filtration with at least two multilayer filter units which are stacked on top of one another, separated by a spacer in each case and connected to one another in a leakproof manner in the vicinity of the edge by means of sealing material which has an annular shape, and with at least one channel for unfiltered material and at least one filtrate channel which connects the filter units.
The invention further relates to a filtration device and a method for producing a filtration module for crossflow filtration.
Membrane filtering modules are frequently used for ultrafiltration, microfiltration or reverse osmosis in the crossflow method of operation in the beverage, pharmaceutical or chemical industries.
What is understood by crossflow filtration is a filtering method wherein the unfiltered material is delivered tangentially to the filtering surface (membrane) as turbulently as possible in order to clear the active membrane layer constantly by rinsing and to counteract the formation of a covering layer, such that the pores of the membranes do not clog prematurely. In this connection solid substances and macromolecules which cannot pass through the membrane are washed away with the tangential flow and are concentrated during the cycle. In this way a residue or unfiltered material is obtained. A small part of the circulating liquid penetrates the membrane in the form of a permeate or filtrate.
A filtering module or a filtering device of the above-mentioned type is known from DE-OS 29 90 253 and DE-OS 34 42 249, for example.
In DE-OS 29 20 253 intermediate layers of flat incompressible woven draining textile are used as spacers between filter units. Filtrate and unfiltered material channels are defined by bores within the edge regions of the filter units which are closed along their entire periphery by a sealing agent.
When the filtering units are constructed in this manner it is inter alia disadvantageous that when sealing the individual filter units in the region of the filtrate or unfiltered material channels on the upper or lower side of the edge regions of the filter units particular care must be taken as to the exact arrangement of the regions which are alternately covered by sealing material or are free.
Likewise, when several filter units are stacked alternately with intermediate grid layers, the openings in the grid woven textile pieces which correspond to those openings in the filter units which are not surrounded at the top or at the bottom by sealing mass, have to be sealed in an expensive manner by introducing additional seals.
If several filter units with intermediate layers of woven textile are to be combined to form a module and if all the openings in the intermediate layer grids and the filter units are to be flush with one another, it is necessary to use a complicated process to glue the stack arrangement along its peripheral edges, especially the outer edge, with a polyurethane-based adhesive or with silicone and thus to seal it.
The high cost of this work, which is often carried out manually, results in high costs for the production of modules of this type. Furthermore, although the selection of a flat gridwork for the spacer layer has the advantage that a plurality of commercially available gridworks is present and these can be easily adapted to the geometrical requirements by punching, the disadvantage resulting therefrom is the relatively high danger of mechanical damage to the sensitive membrane structure by punched grid apertures.
Furthermore, in particular when a fine-meshed woven textile piece is used as a spacer, there is the possibility that under operational conditions a large number of grid points abut on the separating medium and thus the efficiency of the entire arrangement is reduced by a considerable degree.
DE-OS 34 41 249 aims to overcome this disadvantage by bridging the grid regions which are located in the region of the distributing shafts and are to be kept open in the plane of the grid structure with essentially incompressible thin pressure distributing and protective discs for the separating medium. These are constructed in the form of an annular disc, strips, mask or frame according to the respective requirements.
However these discs must again, disadvantageously, be introduced in an additional work stage in order to bridge and seal the relatively large number of apertures in the individual layers which is typical for this type of filter unit.
A second group of publications comprises German Utility Models G 89 05 810 and G 82 31 757 as well as DE-OS 31 27 548.
In G 89 05 810 stacks of membrane pads which are sealed at their outer edges in a flow manner resistant to flow agents are disposed alternately with range spacers. Since the modules are to be used at relatively high pressures, as they are required in the reverse osmosis process, the spacers are constructed in the form of disc bodies, for example of suitable synthetic materials of high rigidity. In order to prevent the membrane of the filter units from being damaged, the spacer elements are provided in a complicated manner with a plurality of elevated projections which project from the surface and on which the membrane pads abut with only a very small contact region. In this way although a very effective surface is gained, practical experience has shown that exactly this type of abutment point can lead to cracks or damage to the sensitive membranes. Moreover, at the low pressures in the ultra- and micro-filtration range a 100 percent sealing effect is not guaranteed with these seals. For this purpose pressures of up to 30 bars are required.
In addition to the passage openings for the flow medium which have to be especially disposed in the spacer, the construction of the filtrate channel is extremely costly since the latter is constructed by the opening of the inner spaces of the membrane pads into a central clamping bolt, wherein the bolt simultaneously fixes the stack and also penetrates corresponding central holes in all the stack elements which have to be disposed separately in each part. This can again only be brought about by using costly additional seals.
G 82 31 757 also proposes a separating plate as a spacer for stacks of filter units, however, the plate still has essentially all the disadvantages explained above in connection with G 89 05 810 with respect to the present invention.
In addition to the disadvantage of high production costs, the special construction of the separating plate also gives rise to the further disadvantage that unacceptable tolerances occur in the thickness of the plastics material plates which are mainly produced by the injection molding process, such that reliable and exact sealing in the region of the filtrate or unfiltered material channels, respectively, is not always provided.
In the case of DE-OS 31 27 548 incompressible supporting plates with channel grooves and apertures are premounted alternately with membrane filter units which are sealed at the edge side, to form a stack, and a free-flowing sealing agent is injected into the communicating sealing grooves and channels, which agent hardens in the compressed stack and in this way firmly combines it to form a stable box.
On the whole the disadvantage of a module of this type of construction is again the costly spacer plate. A further disadvantage is the sealing with a sealing material which has to be injected since the latter spreads in a non-uniform manner through the grooves and channels and thus leakages can lead to rejects.
The boxes produced in this manner are altogether incompressible and in a disadvantageous manner are additionally clamped between casing plates with main connections by means of a plurality of clamping bolts which have to be tightened by means of torque wrenches. Between several boxes mask seals of silicone or polyurethane are used which, however, cannot compensate the finishing tolerances of the casing plates and have only a limited ability to transfer the pressure of the two casing plates exerted by the clamp bolts to the individual modules.
A third group of documents, DE-OS 33 17 517 and DE-PS 35 07 908, deals with similar separating devices, wherein the prefabricated membrane pads are stacked on top of one another, separated by a spacer.
The filtrate and unfiltered material channels are sealed by plastics material seals which are laid around two permeable rods which extend in the filtrate channel. The entire stack is disposed in an external plastics material casing and is fastened by screws. The module prepared in this manner is again especially inserted in fine steel pipe. Apart from sealing problems which occur in the filtrate channel when the stack is compressed by two rods, in this case the membrane pads are pushed apart at the sides, which leads to an irregular flow. The membrane pads which are stacked in this manner crack at the lateral welding points if a pulsing counter pressure occurs, in the case of pulsation by pumps for example.
In contrast to the prior art the invention is thus based on the object of further developing with simple means a filtration module which can be stacked to form filtration devices such that fewer sealing elements are required for its sealing, and such that it can be produced more easily in a process which can be largely automated, wherein damage to the filter units during operation is avoided.